1. Field of the Invention
The present invention relates to a thin film transistor and a method for manufacturing the thin film transistor, and a semiconductor device and a display device formed using the thin film transistor.
2. Description of the Related Art
In recent years, a thin film transistor including a thin semiconductor film (with a thickness of several nanometers to several hundreds of nanometers, approximately) over a substrate having an insulating surface (e.g., a glass substrate) has been attracting attention. Thin film transistors are widely used for ICs (integrated circuits) and electronic devices such as electro-optical devices. In particular, thin film transistors are urgently developed as switching elements of image display devices typified by liquid crystal display devices and the like. In an image display device such as a liquid crystal display device, a thin film transistor including an amorphous semiconductor layer or a thin film transistor including a polycrystalline semiconductor layer is mainly used as a switching element.
As a switching element of an image display device, a thin film transistor in which a microcrystalline semiconductor layer is used for a channel formation region is known in addition to a thin film transistor in which an amorphous semiconductor layer is used for a channel formation region and a thin film transistor in which a polycrystalline semiconductor layer is used for a channel formation region (see Patent Document 1).
Further, there is a method in which characteristics of a thin film transistor are improved by exposure of the thin film transistor to an atmosphere containing oxygen ions and oxygen active species generated by plasma discharge in a gas containing oxygen (see Patent Document 2).